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Daddy-long-legs, or harvestmen, are members of the arachnid order Opiliones, and after spiders are the arachnids most familiar to nonbiologists The species richness of the group is estimated at over 10,000, most of them tropical. The tropical species, in particular, exhibit often bizarre body forms, striking color combinations, startling armaments, and strong chemical defenses. This ancient group can be traced back to 400-million-year-old fossils, which incidentally show them to be the first animals to have evolved a penis. In several Amazonian species, the male builds a nest of mud and is courted by females who vie to lay eggs in the nest, where he defends both the eggs and the young who hatch from them. Many grab their prey with mantis-like palps, but some act as living flypaper, catching small insects on special hairs tipped with blobs of glue. After discussing the above aspects of harvestman biology, the article focuses on the biodiversity of the group, describing key forms, evolution and biogeography.